Motivation

I like her being a barista because:

It fits with the increasingly popular Cozy drinks vibe popularized by Starbucks.
It provides Adelle a social outlet.
1. Which also provides opportunity for her to interact with more plot characters.
It is a good opportunity for Condescension.
It has some potential relationship, either directly or subconsciously, to Adelle as a maid.
It’s unusual for this sort of genre.
Unlike some jobs which can be at least partially performed from home, she will need to commute through the dangerous city to her job location.
It turns the potential location of a Tea shop into an anchor point (by having a major character working there. The first book had a Tea shop which featured in many scenes, but it was still mostly background. Adelle working there gives it more distinction and identity.)

Boss

The Tea shop needs an owner who Adelle reports to.

The owner would need to be a woman.

I can see potential for the owner being either older or younger than Adelle—I see less potential if they are the same age.

If the owner is older, than more likely she would be a respectable character Adelle looks up to.

If the owner is younger, she may be more knowledgeable about the business then Adelle but is also a little immature.

However, I’m picturing Adelle still being fairly young so I can’t imagine the owner practically being much younger than her.